The present invention relates to the practice of field programming memory devices without removing the memory device from the product.
As microprocessor technology becomes more widespread, significantly more kinds of products are being "personalized" to a particular user's application by customized programming of the microprocessor memory. Analogously, as these memory devices become smaller and more cost-efficient, more features of a product are being incorporated into the realm of microprocessor-controlled functions. For example, modern microelectronic technology has permitted a significant increase in the flexibility of features and the number of system parameters designed to be programmed into two-way radios. Synthesizer channels, signalling identification codes, and optional user features are just a few of the microprocessor-controlled system parameters which are currently being programmed into the radio memory.
In the past, this program information has been stored in a fusible-link programmable read-only-memory (PROM) which must be replaced if the system parameters are ever changed. Not only is the removal of the PROM from the product time-consuming and labor-intensive, but also the replacement cost of the fusible-link PROM cannot be overlooked.
Another memory technology has allowed the introduction of the ultraviolet-erasable PROM (UV-EPROM) which may be erased and re-programmed. Although the UV erasable feature has eliminated the replacement cost disadvantage, the UV EPROM must still be removed from the product in order to expose the die to ultraviolet light for erasure. Hence, the reprogramming operation still incurs a relatively large amount of time to remove the PROM from the circuit, to expose the PROM to ultraviolet light for a period of up to thirty minutes to erase the old data, and to reprogram the PROM with new data.
A newer technology, that of the EEPROM (elecrically-erasable-programmable-read-only-memory), has the capability of being electrically erased and re-programmed in-circuit. This feature allows the "personality" of the product to be changed in the field without removing the PROM from the customer unit. In comparison to previous memory technology, the EEPROM has given new life to the electronics industry. EEPROMs offer the numerous advantages that: the EEPROM does not have to be removed from the circuit for programming, so the additional cost of a PROM socket is avoided; factory throughput is increased since "generic" products can be built, tested, and stocked; a wide variety of customer features can readily be programmed into an "off-the-shelf" unit to reduce delivery time; and subminiature inexpensive surface-mount chip memory devices may be utilized for size reduction, since PROM removal is now eliminated.
The implementation of EEPROM technology in portable equipment, such as hand-held radios, radio telephones, or pocket pagers, has encountered several difficulties with the programming operation. First of all, it is highly undesirable to disassemble the radio to connect a programming device to the leads of the EEPROM inside the radio, or to obtain access to the programming circuitry to place the device in the programming mode. Experience has shown that the practice of field-programming the radio by removing the radio covers has been linked to mechanical reliability and/or electrical tuning problems.
Secondly, the alternative approach of providing an externally-available programming connector is not easily implemented in a hand-held compact unit. This approach contradicts the size-minimization, cost-reduction, and minimum-redesign goals of most portable products. Furthermore, it is generally preferable to avoid using any additional externally-accessible connectors on a hand-held electrical device, so as to minimize weatherproofing and static protection problems.
The third approach, that of designing programming circuitry into each product, introduces similar cost, size, and complexity issues. It is readily apparent that a manufacturer of a low-cost pager, for example, would consider it ill-advised to include additional EEPROM programming circuitry inside every unit produced. Moreover, numerous low-cost EEPROMs require a +25 VDC programming voltage to erase and re-program the PROM. In a battery-powered portable radio application, the addition of the programming voltage generating circuitry into each unit is hardly feasible.
A need, therefore, exists for a way to program EEPROM memory inside a portable product that avoids the additional bulk, expense, complexity, and unreliability of supplementary connectors or complicated programming circuitry.